The present invention relates to a microwave oven and, still more particularly, concerns a dual-purpose oven combining heating by UH frequency energy with a traditional heat energy input by heating resistors and convection of the internal atmosphere of the oven.
So-called microwave ovens are already known in the art, where the electromagnetic waves supplied by a suitable source and channelled in a guide which opens into the oven cavity are distributed in the latter by various methods. One of the latter consists in arranging in this cavity or oven muffle, in the vicinity of the exit iris for the waves leaving the guide, a metal-bladed propeller which is driven in rotation and which the microwaves leaving the guide strike, changing phase and being then reflected in directions which vary at each moment because of the relative inclination of the blades and because of their drive. This results in a random distribution of the field in the oven cavity, avoiding the presence of cold and hot regions generated, respectively, by the nodes and the antinodes of the electrical field and producing a more uniform energy distribution. However, this solution which requires the oven to be provided with a specially adapted motor and propeller, appreciably increases the cost of manufacture of the whole unit. Furthermore, the metallic material of the propeller, usually aluminium sheet, undergoes appreciable distortions at high temperature, and this can result in the formation of electrical arcing or, equally, can cause the propeller to jam, this being so especially in so-called pyrolysis ovens where the temperature can locally reach 500.degree. C. or more.
To reduce the cost of such ovens, provision has also been made for eliminating the motor driving the fan purpose oven with traditional cooking, due to an appreciable loss of the heat energy from the oven as soon as the driving air is introduced.
Another traditional solution also consists in providing the oven muffle with a rotating tray supporting the contents to be heated. The waves, which are established in the oven in a nonuniform manner with a succession of hot and cold spots, in combination with the tray rotation, produce a continuous variation in the position of the energy maxima and minima within the contents which, overall, receive an approximately uniform energy. However, here again, the device requires an additional motor for driving the tray, and the fitting of a wave trap on the shaft of the latter in order to avoid the harmful escape of a proportion of the UH frequency energy along this shaft. Moreover, there is a risk that the tray and its drive shaft may distort at high temperature, while the presence of this tray limits the working capacity of the muffle in any case, this being reduced to that of the cylinder whose radius is equal to the distance from the centre of rotation of the tray to the nearest internal wall of the muffle.
Finally, there are known embodiments where the distribution of the flux of electromagnetic waves in the oven cavity is produced by means of a rotating antenna mounted in the region where the guide opens into the muffle, so as to trap the energy leaving the latter and to radiate it into the oven cavity in a more uniform distribution. However, here too, disadvantages continue to exist, resulting from the need to provide a rotary coupling between the cavity and the guide, with the use of a shaft made of dielectric material and driven by a motor outside the guide, the antenna consisting of a bent metal tube associated with a ceramic or similar seal in order to prevent the loss of waves at the exit of this guide. Furthermore, while this device is suitable when the oven operates exclusively on microwave energy, it is not suitable for a dual-purpose oven, in which, in particular, the problems of expansion between the metal and ceramic parts, especially during the pyrolysis stages, lead to difficulties which are considerable and tricky to overcome. In addition, the rotating antenna in such ovens is generally situated in the vault of the muffle in the vicinity of the grill resistor, and this further increases the problem of temperature behaviour. Finally, this device is costly and, once again, appreciably reduces the working volume of the muffle.